An oxynitride glass has a structure formed by the replacement of oxygen in an oxide glass by trivalent nitrogen. It is considered that the replacement leads to a tightening of the glass network by means of the formation of more bonds than those of an oxide glass. Therefore, an oxynitride glass has a high elastic modulus in comparison with a conventional glass.
The compositions of known oxynitride glasses include Ca--Si--Al--O--N, Na--Ca--Si--O--N, La--Si--Al--O--N, Na--B--Si--O--N, Mg--Si--Al--O--N, Si--Al--O--N, Y--Al--Si--O--N, Na--B--Al-- P--O--N and the like.
These oxynitride glasses are produced by, for example, melting, sol-gel, or N.sub.2 -blowing-methods, and by treatment of a porous glass with NH.sub.3 gas.
According to the melting method, a metal nitride is incorporated into a metal oxide and melted at a high temperature. Examples of such a metal oxide include SiO.sub.2, Na.sub.2 O, K.sub.2 O, La.sub.2 O.sub.3, Y.sub.2 O.sub.3, ZrO.sub.2, TiO.sub.2, B.sub.2 O.sub.3 (boron may be considered as a metal, in context). Examples of such a metal nitride include Si.sub.3 N.sub.4, AlN and the like.
According to sol-gel method, an oxide glass precursor having --OH or --OR is reacted with NH.sub.3. Examples of such an oxide glass precursor are silicon tetraalkoxide, titanium tetraalkoxide and the like.
Known oxynitride glass compositions have a nitrogen content of less than 15 atomic percent (9% by weight). That is, a known composition or compounding ratio of components of a oxynitride glass can not replace the oxygen in an oxide glass with nitrogen sufficiently. The nitrogen of a raw material runs away from the glass system during the preparing of an oxynitride glass, so that there is obtained a oxynitride glass having a low nitrogen content (5-6% by weight).
Glass fibers are useful as a reinforcing material. As an effective method for reinforcing a material for a molding such as plastics or cement, it has recently progressed to make a composite material which is composed of said material and a glass fiber. Fibers generally used in such a conventional composite materials are E-glass fibers or S-glass fibers whose strength is not yet sufficient. That is, as to elastic modulus, E-glass shows a maximum modulus of 7,500 kg/mm.sup.2, and Imperial N-672 having the highest elastic modulus shows only 12,110 kg/mm.sup.2,
A known oxynitride glass fiber has been produced by the sol-gel method, and by the method wherein an oxynitride glass bulk is produced and then remelted to give a fiber (see U.S. Pat. No. 4,609,631). A known oxynitride glass fiber obtained by the sol-gel method has an elastic modulus of only about 8,000 kg/mm.sup.2, which is not yet sufficient for reinforcing glass fibers of composite materials. On the other hand, a known oxynitride glass fiber produced by remelting of a bulk oxynitride glass has the maximum nitrogen content of 15 atomic percent (at%) and a high elastic modulus of 140-180 Gpa, but such a fiber is very expensive because it contains 42.6-45.4% by weight (wt%) of yttrium.
The present inventors have intensively studied to obtain an oxynitride glass and the fiber of the same having an excellent strength for reinforcing materials by replacing oxygen in an oxide glass with nitrogen effectively. As the result, it has been found that an oxynitride glass can contain a large amount of nitrogen by increasing the content of CaO, and that a fiber made from such oxynitride glass by melting method has a remarkably high strength and high elastic modulus.